Pipeline emplacements are often subject to various forces, such as hydrodynamic and hydrostatic forces, which can manifest in the displacement of the pipeline from its originally installed position. Consequential pipeline rupture can have adverse environmental and financial repercussions.
Responsible pipeline companies and contractors are committed to installations whose designs provide for a high margin of design tolerance over and above any purely structural requirement for pipeline weighting. This abundance of caution is intended to offset the catastrophic potential attached to taking unwarranted risks that might lead to displacement and consequential pipeline damage, including service interruption, and environmental damage.
The traditional practice in the industry entails weighting the pipeline with massive concrete weights. Earlier designs include clamp-on weights of various descriptions, as well as the more typical large pre-cast concrete arch structures that are arranged in bridging relation over the top of the pipeline. At the same time, they are expensive to fabricate, transport, entail labour intensive installation practices, and generally require increased trench depth and width dimensions in order that their installation can be properly accommodated. Even though these weights are very expensive, the protection they afford in terms of securely anchoring a pipeline offsets their associated materials and installation costs. Although such weights might be used in any number of situations, they appear most commonly in in-ground installations.
One proposed alternative for dealing with the problem of maintaining the positioning of a pipeline within a subterranean emplacement is disclosed in U.S. Pat. No. 3,170,663 (Fite). This patent discloses an anchoring device for a pipeline, which incorporates an arcuate collar that is secured in straddling relation about the upper exterior surface of the pipeline. The collar is held in tensioned relation against that surface by a laterally spaced apart pair of anchoring rods having spiral flights thereon that extend beyond the undersurface of the pipeline and are adapted to positively engage the underlying soil substrate on either side thereof.
Another proposal entails the use, in muskeg environments, of simple sheets of fabric that are intended to be deployed in a pipeline trench, overlaying the installed pipeline. Backfill is then layered over the fabric in the hope that the collected “unit weight” of the resulting overburden will be sufficient to counter any buoyant forces that local ground water might exert on the pipeline. Resort to this approach has been entertained only when alternatives are simply not available, (ie in remote muskeg areas). Moreover, there is a risk that ground water flows will displace some of the “unit weight” of the “disturbed” backfill from above the pipeline. This would be a particular problem in areas where ground surface contours or the grading of the emplacement or a non-level transit of the pipeline, might result in either or both surface and ground water flows that could be channelled within the fabric, almost in the manner of an artificial canal.
More current designs use flexible bag-type weights. These bags are filled with ballast material, such as gravel and coarse sand.
One example of these flexible bag-type weights is shown in U.S. Pat. No. 5,385,430. This patent discloses a flexible bag system with two main compartments which are filled with ballast material, such as gravel and coarse sand, from the area where the pipeline is being laid. Once filled, the weight is positioned over the pipeline, with one compartment on each side of the pipeline, so that the bag straddles the pipeline and its weight will hold the pipeline in position. One disadvantage of this weight is that the main compartment may have excessive bulging when filled with ballast material.
Another example of a flexible bag-type weight for weighting pipelines is disclosed in U.S. Pat. Nos. 8,262,320 and 8,360,688. These patents disclose a flexible bag-type pipeline weight which straddles a pipeline and is filled with ballast material. The bag has two leg portions, extending on either side of a pipeline, and a center section above the pipeline. In the lower portions of the legs are multiple cables or cords extending between the outer and inner leg walls to prevent excessive bulging of the legs. It also includes filling loops and hoisting loops. The filling loops are positioned either on the inner face or the outer face of either the corresponding top section sidewall or the corresponding leg section outer sidewall, at or near the juncture between the top section sidewall and the leg section sidewall.
One disadvantage of this system is that the cables or cords in the legs are cumbersome. The cables or cords may not prevent excessive bulging of the legs when the weight is filled with ballast. Further, the ties are only attached at the lower most portions of the legs and do not prevent excessive bulging of the sidewalls higher up in legs. They may also interfere with ballast from evenly filling the bottom portion of the legs. Further the cables or cords weaken the walls where they extend through the walls. Also, with the ballast material contained in only one large compartment with fluid communication throughout, the safety and purpose of the weight may come under threat if the exterior of the fabric is damaged during the often rough installation process—free flow of the contents would likely occur.
There is therefore a need to provide a weighting bag which is flexible but with a system that prevents excessive bulging of the bag in a compartmentalized manner, can be filled with ballast material and handled easily. There is also a need to provide a flexible bag weighting system which is inexpensive to manufacture.